For many years, workers in the art and science of cell and tissue culture have recognized the importance of growing cells and tissues under conditions that produce results which replicate cells and tissues occurring naturally in animals.
Cultured cells and tissues can be produced in unlimited quantities, and, if they closely imitate the natural product, can be used in many important ways. For example, cultured cells and tissues can be used as reference standards by surgical pathologists when processing specimens of patients' tissue taken for diagnosis of various diseases.
Surgical pathologists are presented daily with puzzles in the form of tissue specimens removed during surgery or various biopsy procedures. The experienced pathologist makes a diagnosis based on the microscopic structure (morphology) of the tissue and multiple clues about its composition. Those clues include specific molecules produced in the tissue through the action of one or more genes, including mutant genes, and they are identified by employing special staining methods to make them visible through a microscope. Those molecules are commonly referred to as “markers”.
When a patient's tissue specimen is submitted to a pathologist, it is first “fixed” by exposing it to a preservative solution, such as formalin. The fixed tissue is then embedded in a small block of paraffin, and the block is sliced with a microtome into thin slices or “sections”, typically four to eight microns thick. Individual sections are attached to standard glass microscope slides, processed to remove the paraffin, and thereafter stained using any of a variety of staining methods known to pathologists. For example, immunostaining is a type of staining in which the tissue section is exposed to a highly specific antibody which binds only to a specific marker in the tissue. The tissue is then treated to produce a colored reaction product at the site of antibody deposition so the presence, location and relative amount of marker in the tissue can be observed by conventional or automated microscopy. However, variations in any step in tissue processing, or staining, or in the chemicals used produce variations in the final staining.
Identification of specific markers is important in the diagnosis and classification of tumors, but without dependable reference standards, interpretation of immunostained specimens is arbitrary and non-quantitative. Variation in results within the same laboratory, to say nothing of variation among different laboratories, limits the utility of this potentially powerful technology. It would be relatively easy to adjust for operational variables, if reliable standard reference tissues were available. However, no single specimen of any naturally occurring tissue is large enough to provide specimens for numerous laboratories repeatedly over long periods. For example, about 30 million paraffin-embedded tissue blocks are prepared each year in the United States alone, and 10–15% of them require immunostaining.
A paper entitled “Three-Dimensional Growth Patterns of Various Human Tumor Cell Lines in Simulated Microgravity of a NASA Bioreactor” by M. Ingram, et al., in In Vitro Cell. Dev. Biol.-Animal 33:459466, June 1997, describes preliminary experiments in which human tumor cells were co-cultured with fibroblast cells. Although those experiments were encouraging, they did not produce tissues which closely replicate natural tissue.